Carboxyl methylation and farnesylation of transducin gamma-subunit synergistically enhance its coupling with metarhodopsin II.

Abstract

A heterotrimeric G-protein in vertebrate photoreceptor cells is called transducin (Tαβγ), whose γ-subunit is a mixture of two components, Tγ-1 and Tγ-2. Tγ-2 is S-farnesylated and partly carboxyl methylated at the C-terminal cysteine residue, whereas Tγ-1 lacks the modified cysteine residue. To elucidate the physiological significance of the double modifications in Tγ, we established a simple chromatographic procedure to isolate Tγ-1, methylated Tγ-2 and non-methylated Tγ-2 on a reversed phase column. Taking advantage of the high and reproducible yield of Tγ from the column, we analyzed the composition of Tγ subspecies in the Tα-Tβγ complex which did not bind with transducin-depleted rod outer segment membranes containing metarhodopsin II. The binding of Tα-Tβγ with the membranes was shown to require the S-farnesylated cysteine residue of Tγ, whose methylation further enhanced the binding. This synergistic effect was not evident when Tα was either absent or converted to the GTP-bound form which is known to dissociate from Tβγ. Thus we concluded that a formation of the ternary complex, Tα-Tβγ-metarhodopsin II, is enhanced by the farnesylation and methylation of Tγ. This suggests that the double modifications provide most efficient signal transduction in photoreceptor cells.